1. Field of the Invention
The present invention generally relates to a screen, an optical film, and a method of manufacturing an optical film.
2. Description of the Related Art
In recent years, overhead projectors and slide projectors have been widely used by speakers at meetings as means for presenting data. Video projectors and motion-picture film projectors, which use liquid crystal panels, have become popular in ordinary households. These projectors employ the following projecting method. Light outputted from a light source is light-modulated by, for example, a transmissive crystal panel to thereby obtain image light. This image light is emitted therefrom through an optical system, such as a lens, and projected onto a screen.
For example, a projector that is capable of forming a color image on a screen has an illuminating optical system for separating light emitted from a light source into red (R), green (G) and blue (B) light fluxes and for focusing each of the R, G, and B light fluxes in a predetermined light path. The projector also has a liquid crystal panel (or light valve) for light-modulating each of the R, G, and B light fluxes separated by this illuminating optical system, and a light synthesis section for synthesizing the R, G, and B light fluxes light-modulated by the liquid crystal panel. A color image synthesized by the light synthesis portion is enlarged and projected onto a screen by a projection lens.
Recently, there has been developed a projector apparatus of the type that spatially modulates each of the R, G, and B light fluxes by using a narrowband three-primary-color light source as a light source and a grating light valve (GLV) in place of a liquid crystal panel.
In order to view an image projected by the above-mentioned projector apparatus, a projector screen is used. Projector screens can be broadly classified into front projector screens, which has projection light irradiated thereon from the front side thereof and with which the reflected light is viewed by a user, and a rear projector screen, which has projection light irradiated thereon from the rear side thereof and with which the transmitted light is viewed by a user from the front side thereof. Of both the front projector screen and the rear projector screen is demanded a wide viewing angle with good visibility.
Therefore, generally, each of the front projector screen and the rear projector screen has a diffusion layer for scattering light, which is provided on the surface thereof The diffusion characteristics of the diffusion layer are the same at the center of the screen as they are in the periphery. The term “diffusion characteristics” as used herein refers to, taking the front projector screen as an example, the characteristics representing the luminance for each scatter angle component of reflection light D91 that is scattered when light is incident upon a certain part O on a screen 91 from the direction of the normal to the surface of the screen 91 (that is, the direction at an angle of 0°) as shown in FIG. 18A. In the present application, the diffusion characteristics are expressed by having the horizontal axis represent the scatter angle of reflection light, with the direction of the normal to the surface of the screen (that is, the front direction of the screen) taken to be 0°, and by having the vertical axis represent luminance. For instance, the diffusion characteristics of a conventional front projector screen are such that the angle at which the luminance of reflection light is greatest is 0° at any part of the screen as shown in FIG. 18B. The diffusion characteristics of the rear projector screen can be expressed in a similar manner.
The diffusion characteristics in the conventional front projector screen are uniform across the entire screen. Therefore, when the screen gain thereof is increased, the difference in luminance between a central part 91b and the peripheral parts 91a and 91c increases as shown in FIG. 19. Thus, the image projected thereon would be-bright at the central part 91b, but darker at the peripheral parts 91a and 91c. In other words, because the incident angle of the projector light on the central part 91b of the screen 91 is 0°, an angular component R91b, whose light intensity is larger than any other angular components of reflection light, is reflected to the side of a viewer 96. However, at each of the peripheral parts 91a and 91b of the screen 91, the incident angle of the projector light is not 0°. Thus, among the angular components of the reflection light, the angular component having the largest light intensity is reflected outward from the screen 91, and only angular components R91a and R91c having a smaller light intensity are reflected to the side of the viewer 96.
This tendency is also present in rear projector screens. As shown in FIG. 20, the projected image is brighter at a central part 92b, but darker at peripheral parts 92a and 92c. 
Various studies have been conducted on the issue of luminance-difference of the screens described above. For example, there has been proposed a front projector screen whose luminance difference is improved by gradually increasing the surface irregularities of the reflection layer thereof from the center towards the periphery, thereby gradually increasing the degree of scattering of the reflected light the closer it gets to the periphery of the screen (see, for instance, Japanese Patent Application Publication No. Hei-10-142699, paragraphs [0016] to [0036], and FIG. 1).
There has also been proposed another screen that includes a group of pairs of mirrors at right angles and in which an even luminance distribution across the screen is achieved by adjusting the angle of inclination of each pair of mirrors (see, for example, Japanese Patent Application Publication No. 2000-162710, paragraphs [0010], [0011], and FIG. 8).
Although the luminance distribution is improved in the front projector screen above in which the surface irregularities of the reflection layer are adjusted, at the peripheral portions, the angular component, whose intensity of reflection light is greatest, is still directed outward. Therefore, there is a considerable amount of reflection light that does not contribute to viewing, light from the projector cannot be used effectively, and consequently, it is difficult to enhance the overall luminance.
Further, in the front projector screen above in which the inclination angle of each pair of mirrors is adjusted, the luminance distribution in the horizontal direction can be made even. However, the luminance distribution in the vertical direction cannot be made even for structural reasons.